1. Using cDNA clones for rat malic enzyme and two other unidentified rat liver messenger ribonucleic acids which are highly responsive to the thyroid hormones, we have probed a genomic library in bacteriophage Charon 4A. Genomic clones for these cDNAs were obtained. 2. A triiodothyronine stimulated rat liver cDNA library and a bovine liver cDNA library were constructed in the expression vector bacteriophage Lambdagt 11.